In order to cause a recording information detection flux or an information recording flux to accurately follow an information track on a recording medium, a tracking servo device is used. It is required to generate a tracking error signal as a servo signal for the servo loop of the servo device. For this, a so-called three-beam method has been used. In this method, as shown in the schematic diagram of FIG. 1 a pair of light fluxes 1 and 3 are irradiated so that they form a pair of tracking beams to be disposed symmetrically with a main beam forming light flux 2 sandwiched therebetween on a line forming a predetermined angle with respect to a recording track T.sub.0 to which tracking is performed. Reflection or transparent light rays of the pair of tracking beams from the recording medium are respectively received by a pair of light reception elements 4 and 5. The difference between the respective outputs A and B of the light reception elements 4 and 5 is calculated by a differential amplifier 6. This difference output, that is A-B, is used as a tracking error signal.
T.sub.1 and T.sub.2 are the recording tracks respectively disposed adjacent to the opposite sides of the target recording track T.sub.0. The track pitch is selected to be T so that the distance between the main beam 2 and each of the tracking beams 1 and 3 in the direction perpendicular to the tracks is T/4.
FIG. 2 is a signal diagram showing the changing state of the signals A and B and the difference signal A-B with respect to an offset of the main beam 2 in the direction perpendicular to the tracks in the device of FIG. 1. Since a substantially linear error characteristic can be obtained within a range of .+-.T/4 of the offset of the main beam 2 with respect to the target track T.sub.0 in the direction perpendicular to the tracks, the output A-B of the differential amplifier 6 can be used as a tracking error signal.
In the case where the track pitch is not fixed and, for example, it is narrow, a sufficient tracking error cannot be obtained in this method. For example, if the track pitch of the right-hand adjacent track T.sub.1 with respect to the object track T.sub.0 is small, the signal characteristic at various portions changes as shown in FIG. 3. The light quantity characteristic of the tracking beam 1 near to the track T.sub.1 takes the maximum value within the range of .+-.T/4 for the offset in the right direction (R) as shown by a curved line A in FIG. 3 and decreases for further rightward offset. Thus, there is a disadvantage that the tracking error signal A-B becomes abnormal for the rightward offset of the main beam 2. Such a disadvantageous phenomenon similarly occurs in the case where the track pitch of the left-hand adjacent track is small with respect to the target track T.sub.0.
FIGS. 4 and 5 illustrate the respective characteristics of the signals A and B at various portions and the difference A-B therebetween corresponding to the cases where the distance between the main beam 2 and each of the tracking beams 1 and 3 in the direction perpendicular to the tracks is smaller and larger than T/4, respectively. In both cases, the tracking error signal A-B also shows a substantially linear characteristic within the range of .+-.T/4. It is required, however, to obtain a linear tracking error characteristic in a range beyond the range of .+-.T/4.